Two-stroke crosshead engines used in marine or stationary applications are equipped with two separate lubricating oil systems. One lubricating system comprises so-called system oil that normally is used for lubrication and cooling of the engine's bearings and e.g. oil-cooled pistons as well as for activation and/or control of various valves or the like. The other lubricating system comprises an all-loss lubricant (cylinder oil) that normally is used for lubrication of the engine's cylinders, piston rings and piston skirt.
In typical two-stroke crosshead engines, the cylinder oil is spent continuously by each turn of the engine whereas the system oil in principle is not spent (except by smaller unintentional leakages). The lubrication system comprising the cylinder oil is also often referred to as an “all-loss” lubrication system as the oil is spent. The use of and various types of both system oil(s) and cylinder oil(s) is very well known in the art.
The cylinder oil typically contains certain additives that function to reduce, minimize or neutralise the acid level of the cylinder system.
Typical cylinder oils usually have an SAE (Society of Automotive Engineering) viscosity equivalent to about 50 and normally have a total base number (TBN) of about 40 to 70 for the neutralisation of acid products produced during the combustion process. Typical system oils usually have an SAE viscosity of about 30 with a relatively low TBN content, typically below 10. These exemplary values may vary dependent on the actual application and the specific design of the systems that the oils are used in.
In recent two-stroke cross-head engine designs involving electronic and/or hydraulic control and/or activation of valves, etc., the minimum performance requirements of the system oil has been substantially increased compared to earlier design using traditional mechanical control/activation.
In four-stroke, trunk piston (diesel) engines, however, typically use only a single oil type for lubrication and cooling. Such engines are used as secondary/auxiliary or propulsion engines on ships, or in stationary power generation or liquid/gas transmission applications. Such used oils typically have a SAE viscosity of about 30 or 40. While the system oil of two-stroke cross-head engines typically remains within its specified performance limits for an extended period of time, trunk piston engine oils are constantly affected by exposure to the combustion process. However, due to the inherent design of two-stroke cross-head engines, spent cylinder lubricants invariably leaks past the piston rod stuffing box contaminating the system oil. Thus, the useful properties of both trunk piston engine and system oil degenerate over time and finally the oils will have to be either replenished or completely changed. Similarly, other lubricants used on-board vessels or at stationary sites, such as hydraulic fluids, gear oils, turbine oils, heavy duty diesel oils, system oils, trunk piston engine oils, compressor oils and the like, do deteriorate over time, due to e.g. contamination, oxidation, hydrolysis etc. and therefore have to be replenished or changed at certain intervals.
The performance level of lubricants is typically measured periodically and may not go beyond certain limits If the oiled component's condition should not be jeopardized. An important cause of performance loss is caused by particle contamination. These particles include combustion by-products and wear components, which can be partially removed by oil separators. However, in the case of two-stroke cross-head engines, one of the sources of contamination is spent cylinder oil leakage past the stuffing box causing both the viscosity and base number of the system oil to increase over time, a process that cannot be reversed by separators.
A diesel engine's frictional loss is mainly of a viscose character. An increase in the viscosity of the system oil will therefore result in a diminished efficiency, increased fuel consumption and increased emissions.
In order to manufacture cylinder oil, prior art methods and systems typically blend suited base oils and suited additives and/or an additive package to obtain a fully formulated cylinder lubricant. This is typically done at a dedicated lubricant blend plant and the resulting cylinder lubricant has to the delivered to a ship or an off-shore plant for use in engines.
Apart from the mentioned inevitable mixing of cylinder oil and system oil prior art methods and systems do not otherwise mix these types of oils. Further, some prior art methods/systems also suggest a variation in lubricant flow rate or properties in response to actual engine conditions, cf. e.g. U.S. Pat. No. 6,779,505. However, such methods and systems do not address the deterioration of oils due to contamination or other processes and the potential to re-use these used oils as cylinder oil.